1. Field of the Invention
This invention relates to improvements in impact printing mechanisms. The improvements are particularly useful in high-speed line printers of the impact type. More particularly, the invention relates to low-mass fast-acting actuators cooperating with low-mass fast-acting print hammers in a novel print head assembly.
2. Description of the Prior Art
Most high-speed impact printers are designed for use as output devices in computing systems. High-speed impact line printers are usually operated by electrical signals originating from a computer or auxiliary equipment to energize actuators which cause print hammers to strike a moving record medium. One class of high-speed impact printers are of the back-printing type wherein type characters are provided on a drum, disk or belt which is moved behind the record medium on which printing is to be effected. Usually an individual type hammer is provided for each of the columns to be printed, and the type hammer is timed to strike or impact the front of the moving record medium when a selected character on the type carrier is opposite the print hammer to effect printing on the moving record medium.
One example of a high-speed impact printer for back printing is provided with a plurality of print hammers in a row opposite a drum on which there is provided a complete set of type on the cylindrical surface of the drum for each print hammer. Accordingly, a row of type having 130 columns would require a drum with 130 individual annular disks of type on the drum. Such drums are expensive and are difficult to balance for high-speed rotation. Heretofore, the fastest rigid type carrier has been limited to speeds below three hundred inches per second.
A single annular disk of type such as that employed on a drum may be placed on a moving frame opposite a single print hammer and traversed across a complete row of type columns to achieve line printing at substantially reduced speeds (usually below a hundred lines per minute).
Impact printers employing moving type require that the print hammer strike the moving record medium and retract immediately to avoid smears caused by the movement of the type and the record medium. When a single disk of type is moved opposite a single print hammer, the type carrier must make one revolution for each columnar position, thus limiting the line printing speed. Band type carriers are arranged to traverse a complete row of type columns each revolution but have other limitations.
When moving type is mounted directly on a high-mass carrier such as a print drum or a print disk, a high-mass hammer may be employed without causing vibrations of the type carrier and it is possible to effect relatively long contact times. When a low-mass flexible band or tape is employed as the type carrier, the band moves on an air film over the surface of the anvil or backup guide which requires the print hammer to force the type carrier through the air film before sufficient pressure is applied to the type to cause printing on a multiple copy record medium. Band type carriers create an air film condition which requires longer contact time than drum type carriers. Increases in contact time increases the tendancy to smear which is normally compensated for by lower speeds. As a general rule, the contact time required for a band type carrier is greater than the contact time for a drum type carrier, and heretofore band type carriers have not been employed in high-speed printers.